In order to disperse liquid into another liquid, the addition of a surfactant (emulsifying agent) is usually required. An emulsifying agent has an amphiphilic molecular structure, which is composed of a polar (hydrophilic) molecular part and a nonpolar (hydrophobic) molecular part, which are spatially separated from each other.
Oil-in-water emulsions used in cosmetics and such stably mix the water-based ingredients and oil-based ingredients using the emulsifying action of the added surfactant. That is, finely dispersed liquid drops of the oil phase are surrounded by shells of the emulsifying agent and the outer phase is the water phase that is the continuous phase; this is said to be the reason for superior texture that gives a dewy fresh tactile sensation.
On the other hand, with the increase in the number of consumers who attach more importance to safety, some of the very sensitive users demand an oil-in-water emulsion that does not have a surfactant that might give irritation or has it in a low enough content to avoid such irritation.
An emulsion prepared by adsorbing powder to the interface, without using a surfactant, is conventionally known as Pickering emulsion.
In the early 1900s, Pickering prepared paraffin/water emulsions that were stabilized by simply adding a colloidal solid such as basic copper sulfate, basic iron sulfate, or metal salts of sulfuric acid. Therefore, this type of emulsion is called a Pickering emulsion. Pickering disclosed the following conditions for the stability of this type of emulsion. (1) The solid particles are suitable for stabilization only when they are significantly smaller than the liquid drops in the inner phase and they don't have a tendency to form aggregates. (2) One of the important properties of the emulsion stabilizing colloidal solid is its wettability. For an O/W emulsion to be stabilized, the colloidal solid is not necessarily more wettable with water than, for example, oil.
Pickering emulsions originally surfaced as unwanted secondary effects in a multitude of situations in industrial processes such as the secondary recovery of petroleum, Bitumen extraction from tar sand, and other separation processes involving two types of non-mixing fluids and fine dispersed solid particles. Therefore, the investigation of the corresponding system such as oil/water/soot or oil/water/slate dust system was the original focus of the research.
Pickering emulsions can be seen in various natural and industrial processes such as crude oil recovery, oil separation, cosmetics, and wastewater treatment.
Many research results have been reported on the preparation of Pickering emulsions (Non-Patent Document 1, for example), and its utilization has been proposed in the perfumery and cosmetics field as well (Patent Documents 1-3).
However, preparation of an oil-in-water Pickering emulsion that can satisfy temperature stability and stirring stability in various environments, which is essential when applying emulsions for perfumery and cosmetics, has been very difficult. For example, in the case of an oil-in-water Pickering emulsion as described above, powder normally is adsorbed on the interface and stably disperses emulsified particles in the emulsion but, when the emulsion is stirred as it is transported and such, the emulsified particles collide with each other and temporarily transform to expose the interface on which the powder is not adsorbed. The exposed interfaces sometimes coalesce to cause aggregation. Therefore, in terms of emulsification stability, conventional oil-in-water Pickering emulsions can hardly be said to be usable as products such as cosmetics.
Recently it has been reported that a stable oil-in-water emulsified composition can be obtained by the combined use of a specific cationic surfactant, polyhydric alcohol, and powder to emulsify the oil phase containing an amphiphilic lipid such as ceramide (see Patent Document 4).
However, Patent Document 4 requires an amphiphilic substance, which forms a liquid crystal structure (a gel) with the surfactant to stabilize the system, but there is a tendency for stickiness at the time of use. Technology to add a very small amount of an amphiphilic substance has been reported (Non-Patent Document 2, for example), but it is difficult to obtain what is sufficiently stable for perfumery and cosmetics; also a new problem arises in terms of the texture during use such as stickiness of the product due to the amphiphilic substance.
Also, Patent Document 5 reports that an oil-in-water emulsion that has superior emulsification stability, is free of stickiness and low in irritation can be obtained by adding specific amounts of powder, oil phase ingredients, water phase ingredients, and a cationic surfactant containing two-chain alkyls. In the invention described in Patent Document 5, it is discovered that, by incorporating the cationic surfactant treatment of the powder into the preparation process of the oil-in-water emulsion, said oil-in-water emulsified composition can easily be obtained.
However, the powder used as the emulsifying agent in these Pickering emulsions is mainly inorganic powder (Patent Document 1: polyalkylsilsesquioxane particles, Patent Document 2: metal oxide, Patent Document 3: silica/titanium dioxide/zinc oxide, Patent Document 4: inorganic powder and such, Patent Document 5: hydrophobized fine particle titanium dioxide, red iron oxide, yellow iron oxide, black iron oxide, and aluminum oxide) and the emulsifying ability of these powders is inferior to that of surfactants, therefore the blend ratio has to be higher than that for conventional surfactants.
As a result, squeakiness and powdery sensation from the use of powder and whiteness after the application cannot be avoided, and therefore most of them are inferior in terms of texture during use.
Patent Document 6 reports Pickering emulsions using spherical organic particles as the emulsifying agent; but the required blend ratio, combined with elastomer-like organopolysiloxane, is 10% or more and the powdery sensation is not quite reduced.
Patent Document 7 reports that a hydrophobic monomer emulsion (Pickering emulsion) can be obtained by using hydrophobin as the emulsifying agent. Non-Patent Document 3 reports that a Pickering emulsion can be obtained by using a flavonoid as the emulsifying agent. However, the use of flavonoids and proteins such as hydrophobin raises the concern of allergies and such, and therefore there are many problems in external preparation applications.
Methods for preparing a corona-core microgel are reported in Patent Document 8, Non-Patent Document 4 and such. Patent Document 9 describes cosmetics into which a corona-core microgel is blended. However, Patent Document 9 proposes the corona-core microgel be a white turbidity agent for providing a white turbid cosmetic. Most of Examples are cosmetics having water-based base agents such as lotions and essences; the corona-core microgel is not used as an emulsifying agent; using a corona-core microgel as the emulsifying agent to prepare an oil-in-water emulsified composition is new. And, in the present invention, a corona-core microgel was found to function as an emulsifying agent, and an oil-in-water emulsified composition superior in stability and texture obtained by using said emulsifying agent is a new invention.